Fracture Of Metallic Materials Research Articles

Modelling the effects of void swelling and spatial heterogeneity on the fracture of metallic materials by crystal plasticity

To theoretically analyse how void swelling and spatial heterogeneity impact the fracture behaviour of metallic materials, a coupled porous crystal plasticity framework has been developed. The proposed framework is then applied to analyse the fracture behaviour of metallic materials under different swelling conditions and types of spatial heterogeneity. It is revealed that at low porosity, the spatial heterogeneity of voids will have limited influence on the fracture behaviour of the material. However, at high porosity, owing to void swelling, a concentrated distribution of voids near grain boundaries can lead to a strong deformation localization, which can be seen as an indicator of possible quasi-brittle fracture. It is further discovered that the inner pressure of the voids tends to reduce the yield stress of the material and promote localized plastic deformation. As a result, such pressure will enhance the impact of concentrated distribution of voids, which can lead to further embrittlement of the material. The present study reveals the key role of void swelling and spatial heterogeneity in regulating the fracture behaviour of metallic materials, a step towards a better understanding over the failure mechanisms of metallic materials under extreme conditions.

Numerical investigation on spall fracture in metallic materials due to laser shock peening via phase field approach to fracture

Laser shock peening (LSP) is an effective surface reinforcement technique for metallic materials. However, when LSP is applied to a thin specimen, an undesirable result, spall fracture, may occur. In this work, the spall fracture generated in multiple-shock LSP processes is investigated via the phase field approach to fracture, and is validated with the experimental result. In particular, the relations between the formation of spall fracture and the shock number, the shock wave pressure, the specimen thickness and the fracture toughness of specimen are studied. The results indicate that the formation of spall fracture in multiple-shock processes is an accumulative process, and is mainly influenced by the shock wave pressure and the specimen thickness, and multiple shocks will complicate the interaction between the tensile wave tail and the compressive wave front of the shock wave, leading to an enlargement of the spall fracture region.

Development and implementation of a micromechanically motivated cohesive zone model for ductile fracture

Gaining popularity after its coupling with the finite element method, cohesive zone modeling has been used extensively to model fracture, especially in delamination problems. Its constitutive relations, i.e. traction-separation laws, are mostly derived phenomenologically without considering the underlying physical mechanisms of crack initiation and propagation. The approach could potentially be used for ductile fracture as well where the micromechanics of the phenomenon is explained by nucleation, growth and coalescence of pores. In this context, the objective of this work is to develop and implement a physically motivated cohesive zone modeling framework for ductile fracture in metallic materials. In order to accomplish this, a micromechanically motivated traction-separation relation which considers the growth of a physical pore is developed. Tractions are directly represented as a function of pore fraction, and its evolution is driven by separations. The model is implemented as an intrinsic cohesive zone element in a two-dimensional setting. Implementation steps and methodology including the finite element framework are presented in detail for mode-I, mode-II and mixed-mode fracture cases. The derivation of the mixed-mode case leads to a novel yield function representation of tractions and separations, instead of an explicit expression. Hence, an incremental implicit elasto-plastic numerical integration scheme is utilized to solve mixed-mode system of equations required for crack initiation and propagation. The framework is implemented as a user element subroutine in Abaqus (UEL) and the numerical simulations are conducted with compact tension (CT) and single edge notch (SEN) specimens to test the implementation and influence of the micromechanical parameters such as pore size, shape and spacing on the ductile crack initiation and propagation. The work is concluded by presenting an outlook for the capability of the model to predict crack path.

Hybrid experimental-numerical strategy for efficiently and accurately identifying post-necking hardening and ductility diagram parameters

Although various effective models for simulating ductile fracture in metallic materials have been established, the methods to identify the required parameters have scarcely been clarified. The difficulty for parameters identification of ductile fracture models is the actual bottleneck for accurate numerical predictions in ductile fracture simulation. In this paper, a strategy to efficiently identify the post-necking strain behaviour and the ductility diagram parameters is proposed by a hybrid experimental-numerical procedure. In the proposed strategy, only two types of experiments are required: a conventional tensile test and a crack growth test. All the parameters to simulate the post-necking strain hardening behaviour and the ductile fracture can be uniquely identified via numerical optimisations using load-displacement curves of the two tests. The proposed strategy was applied to an A2024T3 aluminium alloy and the post-necking hardening and ductility diagram parameters were identified by a flat tensile test and an Arcan test. Two types of plate specimens with a hole were then used to validate the proposed strategy. Good agreements between the experimental results and the numerical predictions clearly show the feasibility and accuracy of the proposed strategy. The results demonstrated that the proposed strategy is simple but effective. Therefore, it can be a general basis for characterising mechanical properties of various metallic materials.

A non-ordinary state-based peridynamic formulation for thermo-visco-plastic deformation and impact fracture

A three-dimensional (3-D) non-local thermo-visco-plastic solid model which considering the plastic hardening, loading rate effect, thermal softening, as well as the fracture behavior of materials was reformulated under the framework of non-ordinary state-based peridynamic theory, and a corresponding spatial integral-type numerical approach was developed together for simulating the impact-caused failure process. They were then employed to study the high-rate thermo-visco-plastic deformation and fracture of metallic materials when subjected to impact loading. The model and numerical approach were validated through simulation of the 3-D Kalthoff-Winkler impacting experiment. The numerical results, including the cracking initiation time and propagation orientation, crack propagation speed, as well as the failure mode and the equivalent stress distribution, agree well with experimental observations and the available numerical results in literature. The effect of the impacting velocity on the crack initiation time, the crack propagation and peak temperature around the crack tips in the target when subjected to impacting loads, were investigated further.

Evolution of Crack Tip Constraint in a Mode II Elastic-Plastic Crack Problem

Numerous studies have shown that crack tip constraint has an important effect on the level of conservatism when crack extension is investigated in elastic-plastic fracture mechanics. Constraint effect has been explored extensively in the past but mainly for pure mode I problems. Very few researchers have dealt with the effects of crack tip constraint on mode II or mixed mode I/II fracture in metallic materials. In this paper, the evolution of mode II constraint parameter Q in terms of applied external load is determined numerically for a test specimen under pure mode II loading. The finite element method is utilized to model the specimen and to study the range of validity of mode II constraint parameter determined from a Q—T diagram. The parameter Q calculated from the finite element simulation (or from the full field solution) is compared with the values of Q determined from the Q—T diagram. For low levels of load, the results of full field solution are shown to be consistent well with the results obtained from the Q—T diagram. However, when the external load increases significantly, the results of Q—T diagram are no longer accurate and mode II constraint parameter Q should be calculated directly from finite element results.

Anisotropic Damage Evolution for Perforated Sheet under Tensile Deformation

It is important to formulate a constitutive equation which represents the growth of voids during plastic deformation in order to predict ductile fracture of metallic materials. For this purpose, we proposed an anisotropic Gurson’s yield function with the damage tensor, which represents the anisotropy due to the void distribution and the damage evolution was assumed isotropic for simplicity. Then we also proposed an anisotropic void growth law derived from the anisotropic Gurson’s yield function based on thermodynamic consideration. In this study we carried out the uniaxial tensile test of perforated sheets of stainless steel and aluminum alloy as the ideal two dimensional model of the damaged material and investigate the damage growth during plastic deformation. As a result, we obtained a good agreement between the experimental and the calculated void growth for both materials and it is also found that material parameters for damage evolutions are almost the same for both materials and are hardly affected by the work-hardening exponent.

Research of fracture of materials and structures under shock-wave loadings by means of the program complex EFES

The results of coordinated experimental and numerical studies of fracture of materials and structures under impact are given in the present paper. Numerical simulation is carried out by the author finite element software package EFES, allowing to simulate a threedimensional setting behavior of complex structures under dynamic loads. Fracture of metallic materials and structures are investigated in the speed range of interaction 50-3000 m/s.

Numerical failure analysis of three-point bending on martensitic hat assembly using advanced plasticity and fracture models for complex loading

A variety of phenomenological models have been developed and validated at a specimen level to simulate plasticity and fracture of metallic materials using finite element analysis. It is both scientifically challenging and crucial for the industry to confirm their applicability at a structural level for the engineering practice. An outwardly simple three-point bending on a hot-stamped martensitic (22MnB5) hat assembly yields complex local loading histories, which eventually creates two symmetric macro-cracks under a semicircular punch. Attempts are made to numerically reproduce the deformed configuration with cracks using the Hosford–Coulomb fracture criterion, recently proposed by Mohr and Marcadet (2015). Two constitutive models of different complexity are compared: pure isotropic hardening with linear damage accumulation and combined isotropic–kinematic hardening with non-linear damage accumulation (Marcadet and Mohr, 2015). The model parameters are identified based on comprehensive mechanical tests. The latter model turns out to predict the location and shape of cracks with great accuracy by considering the dominance of kinematic hardening and enhanced ductility upon loading reversal. The force-punch stroke curves are compared with the experiment. Parametric studies are performed on the friction coefficient and possible anisotropic fracture properties. The deformation sequence is also analyzed.

Modeling ductile fracture using a simplified meshfree method

A new meshfree method for modeling ductile fracture in metallic materials is presented. The approach combines the Gurson–Tveergaard–Needleman (GTN) model and the meshfree cracking particles method (CPM). The GTN model is employed to model the ductile constitutive behavior in the bulk material while the discontinuous CPM is employed at post-localization. In the CPM, fracture is modeled by a set of crack segments that cross the entire domain of influence of the meshfree shape functions. This facilitates the method as no crack topology and associated crack propagation algorithms need to be implemented. A seamless transition from a continuum to a discontinuum is provided by the loss-of-material-stability criterion. The efficiency and accuracy of the method is shown by two problems.

The Fatigue Fracture Criterion Based on the Latent Energy Approach

The extensive literature on the fatigue problem, published for more than one hundred years, is reviewed by the known scientists [1,2]. As it follows from these investigations, the fundamental amount of failures in engineering practice connected with the fatigue fractures of materials and structure elements. The fatigue problem is complicated one and it is not solved yet. So the theoretical and experimental investigations of this problem will be continued. In our paper the energy approach to formulate the fatigue strength criterion is proposed. The criterion is based on the conception of the latent energy [3-7]. This conception was not applied previously to the fatigue problem. The latent energy is consumed to generate the irreversible deformation and to damage and fracture of metallic materials. So the fatigue fracture criterion can be formulated using the results of latent energy measurements in the macro experiments. This is most impotent advantage of the proposed approach. The logistic function is used to describe the dependence of latent energy from the value of irreversible deformation. It is assumed that the cyclic strength of metals is defined by the latent energy, stored in specimen, when it is reached the critical value in accordance with the logistic curve in a saturation zone. This proposal is used to formulate the fatigue strength criterion. The functions and parameters of received criterion are concretized and comparisons with experimental results for axial cyclic tension for sheet aluminum alloy specimens are given.

Energy consumption for creep fracture of metallic materials

The energy conservation law is applied to formulate the ductile and brittle creep fracture criterion for metallic materials. The criterion contains a summary of heat and latent energies. Assuming that the heat energy is given out so it has no effect on the fracture process, the ductile creep fracture criterion is simplified. To take into account the evaluation of the damage state of materials the compressibility condition is introduced and the brittle creep fracture law is formulated.

Laws of fatigue fracture of metallic materials

A kinetic criterion of fatigue fracture of metallic materials is suggested and used for showing the existence of a single fatigue curve for cases of soft (with specified load amplitude) and rigid (with specified strain range in a cycle) loading for the stages of crack formation and propagation. The criterion makes it possible to compute the endurance of elements of a structure and levels of accumulated damage in terms of the ultimate accumulated damage for any loading stage including programmed loading (combination of soft and rigid loading, double-frequency and random loading, and loading with temporary holds under load). The criterion is substantiated by numerous experimental data for various metallic structural materials.

Structural mechanisms of low-temperature work hardening and fracture of cryogenic metallic materials

On the basis of the results of original research, various physical aspects of work hardening and fracture of metallic materials at low temperatures (the macroscopic behavior, microstructural processes governing plastic flow and work hardening, the mechanisms of nucleation and propagation of fracture in the final stage of plastic deformation) are analyzed from a unified point of view: the evolution of the microstructural state during plastic deformation of different groups of structural metallic materials in the temperature range 300–4.2 K. The characteristic physical situations realized in low-temperature plastic deformation and fracture of solids are generalized and classified.

Mechanistic fracture criteria for the failure of human cortical bone.

A mechanistic understanding of fracture in human bone is critical to predicting fracture risk associated with age and disease. Despite extensive work, a mechanistic framework for describing how the microstructure affects the failure of bone is lacking. Although micromechanical models incorporating local failure criteria have been developed for metallic and ceramic materials, few such models exist for biological materials. In fact, there is no proof to support the widely held belief that fracture in bone is locally strain-controlled, as for example has been shown for ductile fracture in metallic materials. In the present study, we provide such evidence through a novel series of experiments involving a double-notch-bend geometry, designed to shed light on the nature of the critical failure events in bone. We examine how the propagating crack interacts with the bone microstructure to provide some mechanistic understanding of fracture and to define how properties vary with orientation. It was found that fracture in human cortical bone is consistent with strain-controlled failure, and the influence of microstructure can be described in terms of several toughening mechanisms. We provide estimates of the relative importance of these mechanisms, such as uncracked-ligament bridging.

Use of the Laws of Delayed Fracture of Metallic Materials for Forecasting the Remaining Life of Structures under a Test Load

Use of the Laws of Delayed Fracture of Metallic Materials for Forecasting the Remaining Life of Structures under a Test Load

Damage mechanical investigations at ductile fracture of free-cutting steels by means of microscopy - void growth and fracture surface topography

At ductile fracture of metallic materials the growth of micro-voids generated by particles in the metallic matrix as for example manganese sulfides can be observed as the dominating damage mechanism. In order to measure and model this damage usually damage parameters such as for example the void radius are used. The final size of grown voids at crack initiation can be estimated by measuring the dimple sizes on the fracture surfaces. Beside these values topometric parameters were applied to characterise the fracture behaviour of the material by its fracture surface. The final void sizes at crack initiation measured on the fracture surfaces partly show a decreasing tendency with higher stress triaxiality, that means a competition between stress state and void growth can be observed. The measured void sizes are generally comparable to values from model calculations. Using the fractal dimension it is possible to distinguish fracture surface topographies resulting in materials showing different fracture toughness values.

Microscopic and Mesoscopic Evaluations of Materials. Observation of Fatigue Crack Initiation Process in .ALPHA.-Brass by AFM.

Since the surface morphology of materials can be observed with atomic-scale resolution, scanning atomic force microscopy (AFM) is a powerful technique to study mechanisms of fatigue and fracture of metallic materials. In the present study, slip band formation and fatigue crack initiation processes in α-brass were observed by means of AFM. Surface of a fatigued specimen was observed under maximum stress. unloading state, and minimum stress at fatigue cycles. In the initial stage of the fatigue process, slip bands which formed only under tension stress or compression stress were observed. These kinds of slip bands, however, disappeared shortly. Under tension stress, cracks could be detected easily just after initiation. For specimens at the unloading state, cracks just after initiation were hardly detected from the AFM images, By measuring the size of slip bands at the unloading state, however, crack initiation from slip bands could be detected. Before crack initiation heights and width of extrusions and the depth and width of intrusions gradually increased with the number of cycles. After crack initiation, one of these values changed drastically, depending on the slip band angle relative to the stress axis and the shape of slip bands.

Time-Dependent Small-Scale Fracture in Metallic Materials at High Temperatures

This paper deals with time-dependent small-scaleintergranular fracture in metals at high temperatures. First, diffusional cavitation during fatigue isdiscussed in terms of its mechanism. Creep cavitiesand cavity-type cracks nucleate and grow inside thematerial under combined very slow tension and fastcompression cycling at relatively high temperatures,and they are extinguished by the succeeding fatigueunder the inverse cycling of combined fast tension andvery slow compression. Second, small-scale fractureis discussed in terms of its process. Fracture of athin aluminum wire in micro-electronic packages isdemonstrated through a numerical simulation of grainboundary grooving caused by surface and grain boundarydiffusion. Third, cavities and microcracks appearingin polycrystalline steels are discussed in terms ofthe effects of their microstructures. Characterization of multiple microcracking isattempted on the bases of a numerical simulation.

Evaluating the reliability of predictions of the rupture strength of metallic materials for long service lives

It is shown that characteristics of plastic strain in the case of delayed fracture (DFPS) under creep conditions reflect fundamental laws governing the temperature- and time-dependent fracture of metallic materials if these characteristics are functions of stress and temperature. The laws governing the change in DFPS are different for different classes of metallic materials. For the unalloyed steels studied in the present investigation, DFPS characteristics increase with a decrease in stress and then — if the time to fracture is very long — decrease. In the case of chromium-nickel steels, a decrease in stress (an increase in time to fracture) is accompanied by a decrease in these characteristics. However, this decrease slows after the passage of certain stress regions which exist for each test temperature. Two laws of development of DFPS are established for the above steels in tests lasting 100,000 h or more. The rupture strength of these steels for times greater than 100,000–200,000 h can be reliably predicted only with allowance for the kinetics of damage accumulation in the material, which is qualitatively reflected by the laws which govern DFPS development in relation to stress and temperature. Numerical solutions are presented for prediction problems for periods of 300,000 h or more. The reliability of the solutions is evaluated.